Sequence stratigraphy and lithofacies paleogeographic evolution of Katian Stage-Aeronian Stage in southern Sichuan Basin,SW China

Based on the lithologies,sedimentary structures,graptolite zones,inorganic geochemical characteristics,electrical data of 110 shale gas wells in southern Sichuan Basin and the mineral quantitative analysis technology of scanning electron microscope,the stratigraphic sequences of the Upper Ordovician Katian Stage-Himantian Stage-Silurian Rhuddanian Stage-Aeronian Stage are divided,the sedimentary characteristics and fourth-order sequence evolution are analyzed.The target layer can be divided into two sequences,namely SQ1 and SQ2.According to Ordovician-Silurian sedimentary background,the gamma value of the target layer and U/Th,5 maximum flooding surfaces and 12 system tracts are identified.According to system tracts and their combinations,eight fourth-order sequences are identified,namely,Pss1-Pss8 from old to new.The development period and scale of dominant shale facies from Katian stage to Aeronian stage in southern Sichuan are restored.The best-quality dolomite/calcite-bearing siliceous shale facies,siliceous shale facies,clay-bearing siliceous shale facies and feldspar-bearing siliceous shale facies mainly occur in Pss3-Pss5 of Weiyuan,Western Chongqing and Luzhou,Pss6 of Western Changning-Northern Luzhou-Central Western Chongqing and Pss3-Pss4 of Changning.The siliceous clay shale facies second in quality mainly occurs in Pss6 of Southern Luzhou-Changning area(excluding Western Changning area),Pss7 of Eastern Weiyuan-Northern Western Chongqing-Southern Luzhou and Pss8 of Northern Luzhou-Weiyuan-Western Chongqing.The fourth-order sequence evolution model of Katian stage-Aeronian stage in southern Sichuan is established.During the depositional period of Pss1-Pss8,the sea level had six regressions and five transgressions,and the first transgression SQ2-MFS1 after glaciation was the largest flooding surface.